1. Field of the Invention
This invention relates in general to line interface devices, and more particularly to a low power CMOS line driver having dynamic biasing capabilities.
2. Description of Related Art
Digital systems typically include several Very Large Scale Integrated (VLSI) circuits that cooperate and communicate with one-another to perform a desired task. Data transceivers are used to interface the VLSI circuits to the transmission medium.
A data transceiver is a read/write terminal capable of transmitting information to and receiving information from the transmission medium. A transceiver typically includes a line driver stage (or simply "driver") and a receiver stage (or simply "receiver"). The common purpose of transmission line drivers and receivers is to transmit data quickly and reliably through a variety of environments over electrically long distances. This task is complicated by the fact that externally introduced noise and ground shifts can severely degrade the data.
Drivers may amplify digital signal outputs from the VLSI circuitry so that the signals can be properly transmitted on the transmission medium. Receivers are typically differential amplifiers that receive signals from the transmission medium and provide outputs to the VLSI circuitry that are representative of digital information received from the medium.
Conventional drivers usually include level shifting capability to provide compatibility with different integrated circuit technologies. Specifically, before a driver transmits a signal across a transmission medium, the driver may change the nominal voltage swing (or the "dynamic signal range") utilized by the VLSI circuitry, e.g., CMOS, TTL, ECL, etc., to a different voltage swing that is utilized by the transmission medium. Thus, a driver may amplify a digital signal, as well as changes the nominal voltage swing of the signal as well.
CMOS technology is attractive for implementing VLSI circuits with high density and with much lower power dissipation than its bipolar counterpart. However, standard TTL or CMOS circuits operate between 5 Volts and ground which causes them to dissipate excessive amounts of power when driving terminated transmission lines. ECL has been used for many years to drive terminated transmission lines; however, ECL has relatively high power dissipation.
A different nominal voltage swing is normally used when transmitting data across a transmission medium in order to conserve power. Specifically, the power internally dissipated by the driver is proportional to the nominal voltage swing of the binary signal it applies to the transmission line. Therefore, power dissipation is reduced if the driver transmits a signal having a relatively small voltage swing over the transmission line.
Low power CMOS line drivers are needed to meet the low power, high speed and high return loss requirements for quad E1(2.048 MHz) and E2(8.448 MHz) transceivers. Fast transition edges of E1/E2 pulses require a line driver to have a high slew rate. Further, to get a high return loss, the line driver's output must exhibit low impedance.
It can be seen that there is a need for a CMOS line driver that can provide the high slew rate required for initial pulse edges with low power consumption.
It can also be seen that there is a need for a low power CMOS line driver with a high slew rate and a low output impedance.